The 12 dumbest things you can do with your money in your 30s — and how to avoid them

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Time is still on your side in your 30s. If you manage your money well, it can mean huge financial gains in the future.

That's easier said than done. And many of us tend to assume that we'll have more money in our 40s and have plenty of time to tune up our finances, which can lead to some poor money decisions.

Here are 12 of the worst money decisions — and how to avoid making them.

1. Not taking advantage of work benefits.

If you're not taking full advantage of your employee benefits, you're leaving money on the table. Some of the more overlooked, yet incredibly advantageous, benefits include:

Healthcare flexible-spending account: This type of account is a pretax benefit account you can use to cover a variety of healthcare products and services, from acupuncture and physical therapy to vaccines and over-the-counter medicine (note that OTC medicines are only eligible when prescribed by a physician). You can put up to $2,550 of tax-free money into this account in 2015, and save about 30% on healthcare expenses with the tax break, WageWorks reports.

Dependent-care flexible-spending account: If you have young children, dependent-care FSAs are worth considering. This account works similarly to the healthcare FSA, in that you can contribute pretax money, but is specific for dependent-care services, such as preschool, summer camp, daycare, or before- and after-school programs.

Commuter benefits: These are often overlooked, but they can save you over $600 each year, WageWorks tells The Wall Street Journal. The concept is simple: Employees can use pretax money from their paychecks to cover mass-transit passes — including the train, subway, bus, ferry, and parking.

It's worth it to research and talk to your human-resources department to understand the scope of what's available to you, as these benefits could save you thousands of dollars each year.

2. Not increasing your 401(k) contributions.

In your 30s, one of the greatest things you have going for you is time, and the more money you can save at an early age, the greater the dividends will be down the road. You should already be contributing to your employer's 401(k) retirement account and be taking full advantage of any available company match program, but if you get a pay raise or bonus, increase that contribution.

Also, get in the habit of upping your contribution at the end of each year, even if it's just 1%. Check online to see if you can set up "auto-increase," which will automatically increase your contributions every year.

3. Only putting money toward a 401(k).

By the time you're in your 30s, you should be considering other investment vehicles beyond your 401(k) plan. A good next step is to put money toward a Roth IRA, another retirement-savings vehicle that offers tax benefits and is particularly well suited to younger people who earn less than the income cap — $116,000 a year or less for individuals, $183,000 or less for married couples filing jointly.

Contributions to this type of fund are taxed when they're made, so you can withdraw the contributions and earnings tax-free once you reach 59 1/2.

If you still have money left over, you can research low-cost index funds, which legendary investors Warren Buffett and Jack Bogle recommend, and look into the online investment platforms known as "robo-advisers."